Radiation counting (photon counting) performed to count a radiation dose incident on a detector while performing individual energy separation in an incident photon unit is currently applied in various fields such as dosimeters and gamma cameras. A representative example is a broad dosimeter typified by a survey meter. As a detector, a scintillator and a photomultiplier tube are normally used. The energy and number of radiation beams incident on the detector are counted. When one or more photos of radiation are incident on a scintillator, the scintillator emits light and a pulse of the visible light with a light amount proportional to energy of radiation is released. The pulse of the emitted light arrives and is detected by a photomultiplier tube whenever a photon of the radiation is incident. Here, the scintillator is covered with a partition wall in a state in which only a surface oriented to the photomultiplier tube is open. The partition wall blocks infiltration of the visible light from the outside and preferably reflects light generated from the inside to cause all of the light to be incident on the photomultiplier tube.
In the dosimeter, the photomultiplier tube generates an analog electric pulse by converting a pulse of emitted light into electrons and amplifying the electrons. A pulse height of the analog electric pulse is proportional to a light amount of the emitted light of the scintillator, that is, energy of radiation. An independent pulse is output whenever one photon of radiation is incident. Therefore, the dosimeter can obtain the number of photons of incident radiation by counting the number of pulses.
In the above-described dosimeter, a detection circuit includes, for example, an amplifier, an integrator, and analog-to-digital (AD) converter. The amplifier further amplifies an output analog signal, the integrator integrates pulses, and the AD converter performs AD conversion. Thus, the dosimeter can derive energy of each photon of incident radiation as a digital value. A digital processing circuit in the dosimeter accumulates an output result of the detection circuit during a predetermined period and derives an energy spectrum of the photons of the radiation. The energy spectrum indicates a presence ratio of the photons of the radiation captured by the dosimeter for each energy. Thus, the dosimeter can specify a radiation source. A transmission probability or an acquisition probability of the radiation further captured by the scintillator differs for each energy. Accordingly, when the number of photons acquired for each energy is returned as the acquisition probability by the digital processing circuit, the number of incident photons can be obtained. In this way, a dose is corrected in accordance with a G function or Dyson boson mapping (DBM) (for example, see Patent Literature 1).
Using a scintillator and a photomultiplier tube in the above-described radiation photon counting is mainstream. However, the photomultiplier tube is expensive and is not appropriate for miniaturization and light reduction. In addition, there is also problem of the photomultiplier tube being easily affected by magnetic fields. Instead of the photomultiplier tube, use of an avalanche photodiode (ADP) or an array of silicon photomultipliers (SiPM) has also been proposed. However, for the former, an output signal is considerably weak, an output fluctuation caused by temperature is severe, and an influence of an external environment is great. In addition, for the latter, there is a problem of a dark current being large since a high electric field is necessary and floor noise being large due to an afterpulse, crosstalk, or the like. Further, since an APD and SiPM use high voltages, a separate power supply circuit is necessary and an output is also an analog signal. Therefore, it is necessary to externally provide a separate amplifier, integrated circuit, or AD conversion circuit, and thus there is problem of an influence of external noise during signal transfer.
On the other hand, Patent Literature 2 proposes a new image sensor using photon counting in which a dynamic range is raised using time division and surface division by a plurality of pixels together while following a circuit configuration of a complementary MOS (CMOS) imager. Such a device can also be used as a photon counting device in which an entire pixel array in a chip is configured as one light reception surface. In the above-described image sensor, an AD conversion circuit is mounted as an on-chip, a pixel signal is received, and whether a photon is incident on each pixel is subjected to binary determination by providing a threshold.